This invention relates to an exhaust system for a four cycle internal combustion engine and particularly for such an engine as utilized in the propulsion device for an outboard motor.
Although two cycle engines have been conventionally employed as the power plants in outboard motors due to their simplicity and high specific output, environmental and other concerns are creating increased interest in utilization of four cycle engines for these applications. In the interest of obtaining high specific outputs, the four cycle engines utilized for outboard motors should have overhead valves and preferably overhead camshafts and possibly even multiple valves per cylinder. Although this somewhat complicates the structure of the engine, it does assist in achieving higher efficiencies and better performance for a given size package.
Certain difficulties arise in connection with the application of four cycle engines for outboard motors. These arise in part, because of the fact that the engine in an outboard motor normally has its crankshaft rotating about a vertical rather than a horizontal axis. One of the components of the engine that is affected by this disposition is the exhaust system.
The design of exhaust systems for outboard motors is further complicated by the fact that the space available for silencing is quite limited. Thus, it is generally the practice to collect the exhaust gases from the engine and deliver them back to the atmosphere through an exhaust system that is formed in major part in the drive shaft housing and lower unit. This gives space for silencing devices such as expansion chambers and the like. In addition, the exhaust gases are discharged under normal running conditions through an underwater exhaust gas discharge.
Thus, if a four cycle engine and particularly an overhead valve four cycle engine is employed as a power plant in an outboard motor, there must be provided an exhaust system for the engine that will collect the exhaust gases and deliver them in a generally downward direction to the driveshaft housing and lower unit.
Although external exhaust manifolds can be employed for this purpose, the space limitations and the desire to obtain cooling for the exhaust system makes it desirable to discharge the exhaust gases actually through the engine body rather than through a separate exhaust manifold that is attached to the engine body.
There has been proposed, therefore, an arrangement where the cylinder head exhaust passages exit the combustion chambers and then curve in a re-entrant fashion so as to exit the cylinder head through openings formed in a surface of the cylinder head that extends parallel to the surface that engages the cylinder block to form the combustion chambers. With this type of arrangement, the cylinder block itself is formed with an exhaust collector section that is comprised of runner sections that mate with the cylinder head exhaust passage outlets and a collector section that extends generally vertically downwardly to communicate with an exhaust system provided in the driveshaft housing and lower unit.
In order to provide smooth flow of the exhaust gases and generally unrestricted flow, the cylinder block exhaust manifold is formed with curved runner sections that curve into the vertically extending collector section. Although the use of the curved runner sections provide a smooth flow, it offers significant problems in actually forming the cylinder block. It is desirable to avoid machining operations and the use of die casting techniques also is desirable. Such curved sections are, however, somewhat difficult to form in most die casting equipment.
It is, therefore, a principle object to this invention to provide an improved exhaust system for a four cycle engine as applied to an outboard motor.
It is a further object of this invention to provide an improved exhaust system for an overhead valve multi-cylinder engine wherein the exhaust manifold is formed in the cylinder block.
It is a further object of this invention to provide a cylinder block exhaust manifold for an engine that lends itself to outboard motor application and wherein the runner section can be conveniently and simply formed and will nevertheless offer a smooth and unrestricted exhaust gas path from the combustion chambers to the engine exhaust system.